Patent Application
20090277441
1. Field of the Invention (Technical Field)
The present invention relates generally to systems for solar heating and processing of gases, particularly to heat exchangers for use in such systems, and more specifically to a heat exchanger for use in association with a solar energy collector, such as a focusing dish, to convert solar energy into thermal energy to process gases or to drive, for example, a Brayton Engine.
2. Background Art
One of the most critical challenges confronting mankind is that of dwindling sources of non-renewable energy. Consequently, a wide variety of increasingly sophisticated and promising efforts have been, and are, made in the field of solar energy. Devices and methods for exploiting solar energy as a renewable resource fall into at least two general categories: those attempting to convert solar energy directly into end-use energy (such as photovoltaic electricity generation, and passive thermal heating of dwellings), and those seeking to harness solar energy as an intermediate energy source for processing feedstock into end-use fuels (e.g., methane).
There are known apparatuses and methods for exploiting solar energy to process feedstock gasses to generate directly useable energy and/or derivative fuels. Two examples which serve as background to the present disclosure are the systems and methods of U.S. Pat. No. 6,066,187 to Jensen, et al., entitled “Solar Reduction of CO2,” and U.S. Pat. No. 7,140,181 to Jensen, et al., entitled “Reactor for Solar Processing of Slightly-absorbing or Transparent Gases,” both of which name a co-inventor in common with the present application. The disclosures and teachings of these two patents are incorporated herein in by reference.
In the former '087 patent to Jensen, et al., the red shift of the absorption spectrum of CO2 with increasing temperature permits the use of sunlight to photolyze CO2 to CO. The disclosed processes of the '087 patent to Jensen, et al., include: preheating CO2 to near 1800 K; exposing the preheated CO2 to sunlight, whereby CO, O2 and 0 are produced; and cooling the hot product mix by rapid admixture with room temperature CO2. The excess thermal energy may be used to produce electricity, and to heat additional CO2 for subsequent process steps. The product CO may be used to generate H2 by the shift reaction or to synthesize methanol.
In the latter '181 patent to Jensen, et al., there is disclosed a solar-powered reactor for processing of slightly absorbing and transparent gases to providing storable, renewable, energy through solar dissociation of gas molecules. The dissociation products are the precursors readily useable and-use liquid and gaseous fuels, such as hydrogen and methanol/ethanol. An apparatus and method using a solar concentrator (such as a focusing trough or dish) directed at the receiving end of a reactor are disclosed. A range of designs of reactors for the dissociation of gases, both those that absorb slightly in the visible spectrum and those that are transparent in the visible and only absorb in the infrared, are described.
The methods and apparatuses of the foregoing two patents, however, involve the heating of the process gases to over 2,000 degrees C., complicating the design, and increasing construction costs, for functional reactor systems. It would be desirable to provide a solar-energy base system for generating useable energy, particularly derivative storable fuels such as methane, but which does not involve such relatively high operating temperatures. More specifically, lower operating temperatures. (e.g., around 800-1100° C.) might be coupled with higher gas through-put, but at comparatively lower entropies, to permit the generation of directly exploitable energy, or for the reformation of methane as an end-use storable/portable fuel, or other hot gas processing.
Against the foregoing background, the present apparatus and method were conceived and reduced to practice.
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings:
Like numbers are used to denote like elements and components throughout the various drawing figure views.
The present disclosure pertains to an apparatus and method for solar heating gases, such as to heat a feedstock gas to drive an engine, or to heat a gas for processing, for example for methane reformation or to drive a reverse water-gas shift. There is provided by the disclosure a processor by which a feed or process gas, which can be a gas relatively transparent to much of the solar spectrum, can be effectively heated by solar energy so that the hot gas can be harnessed to drive an engine, or for use in the production of useable fuels.
Succinctly, the presently disclosed apparatus and method allow for collected solar energy to be transferred by convection into a flowing feed or process gas. Convective heating of a flowing gas is, of course, well-known in the art of heat exchangers generally, but the present invention enables the efficient exploitation of solar energy to heat a flowing process gas. Among advantages of the present invention is its capability to provide a very high heat exchanger surface area to maximize the transfer of solar power to a flowing gas. The apparatus maximizes the area A factor in the convective heat transfer equation:
P=K A ΔT
where P is the thermal power transferred, K is the convective heat transfer constant, and ΔT is the temperature difference between the gas temperature and the temperature of the surface. The area A is maximized, yet in a manner which permits solar energy directly to heat that surface area by direct impingement.
The apparatus and method can be harnessed for driving a heat engine, for example a Brayton Engine (e.g., in the form of a Capstone 30 engine), with the energy received from a solar energy focusing dish. There is disclosed hereby a gas processor apparatus including a heat exchanger capable of transferring 70 kW of solar energy into heat, into a relatively high flow gas, to drive for example a Brayton Engine. The low-entropy processor assembly, including a beneficial foil-coil heat exchanger, is depicted generally in the drawing figures and will be described. It is contemplated that one embodiment of the the solar-powered apparatus and method according to the present disclosure is capable, for example, of heating 200 cubic feet per minute of gas (Cp=7.5 cal/mole deg) approximately 500 degrees C. in a single pass. The single-pass methodology nevertheless can result in final temperatures of about 900-100° C. Such a gas temperature range is not only excellent for Brayton engines, but also for, for example, methane reforming.
An apparatus generally according to the present disclosure has been operated to drive a Capstone 30 generator. A suitable gas flow rate according to one possible system according to this disclosure is 150 to 170 SCFM, rather near a target 200 CFM at 70 kW. The incident solar power on a typical run might be near 50 kW.
The present apparatus and method also are well-suited for chemical gas processing, such as reforming of methane. That utility was as a secondary use in prior U.S. Pat. No. 7,140,181, but it has been determined that the design of that disclosure is “overkill” for reforming methane. The presently disclosed simpler, higher through-put, lower entropy production design is more advantageous for heating at to around 900° C. Also, the corrugated foil coil hot body exchanger disclosed hereby lends itself to catalytic reforming.
Attention is invited to
Referring still to
The major components of a possible and basic gas processor assembly 20 according to the present disclosure are shown in the exploded view of
The proximate end 29 of the housing 24 is the aperture or mouth end, which is that end which faces toward the solar collector 10. The solar collector 10 directs collected energy toward the proximate end 29 of the processor assembly 20, for passage through the proximate end of the housing 24. When assembled, the processor assembly 20 includes at least one quartz pane 28 for closing the proximate end of the housing 24. Quartz pane 28 is configured according to principles known in the art, and permits transmission there-through of solar energy directed from the solar collector 10. More than one quartz pane may be provided in parallel relation, so as to provide an outer pane on the exterior of the processor assembly 20 and a second pane interiorly to the first. It is known that a quartz pane causes some losses (e.g., about 10%) in transmitted solar energy, but quartz is suited for the high temperatures involved while permitting operation of the processor by solar power. The quartz pane 28 configured according to known principles is mounted on or about the proximate end of the housing 24 according generally to convention, using suitable gaskets or O-rings and attachment means (not shown in
The distal end of the processor assembly 20 is sealably closed using a back panel 34, which may be a stainless steel disc of dimensions appropriate to seal the distal end. Back panel 34 is securely mounted to the housing 24 using known gasket and attachment means (not shown) to prevent leakage of gas into our out from the interior of the processor 20. The entire processor assembly is constructed to operate with an interior pressure, which pressure will depend upon the gas process reaction to be exploited. Thus, the design of the processor assembly 20 will include from known information a construction to permit operation at elevated interior pressures; the Reverse Water-Gas Shift Reaction, for example, may be promoted by pressure conditions well exceeding atmospheric pressure.
In one embodiment, there is optionally provided within the processor 20 near the distal end thereof a mirror 40. Mirror 40 is fashioned from a high-temperature material and is polished to reflect incident solar energy. The mirror 40, if deployed, is situated axially inwardly from the back panel 34, so as to be contained within the distal end of the housing 24.
As seen in
Reference is invited to
Incident solar energy is transmitted primarily by conduction from the front face 52 throughout the body of the exchanger 50. Solar rays also may reflectively “bounce” along the axial flow channels defined through the exchanger (as described further herein), enhancing the direct radiant transfer of solar energy to the exchanger 50. Additionally, the feed or process gas flows axially through the exchanger 50 (in either direction, as described more fully herein), such that solar heat which has been transferred into the gas also is restored to the foils 56, 58 of the exchanger by means of convection. The latter is particularly beneficial in the embodiment of the apparatus shown in
The proportion of heat transfer by conduction versus by radiation depends upon the dimensions of the openings in the face of the exchanger, the gas flow rate, and the thickness of the foils 56, 58 in the exchanger 50. In-foil conductive heat transfer along the exchanger 50 depends upon, among other factors, the composition of the exchanger foils 56 and 58, and their thicknesses. The solar radiation heating along the axis of the exchanger is a function of, among other things, the size of the openings in the front face 52 of the exchanger, and the optics (e.g., F factor or degree of focused convergence) of the incoming solar rays. Applying known physical formulae, it is possible to calculate the in-foil conduction and axial radiation transfer values, permitting some customization of the exchanger construction to optimize its operation.
Such a configuration for the processor assembly offers benefit of fully “ingesting” the incident solar energy. This advantage is complemented by the benefit of a modest-diameter aperture at the mouth end 29 of the processor, thus reducing energy losses from back-irradiation.
Combined reference is made to
The corrugated foil baffle 58 and the separator 56 are fabricated from thin, flexible strips of suitable metal alloy, such as copper allow or nickel alloy, capable of withstanding the high temperatures resulting from the solar energy impinging the exchanger 50. By way of example, the foil strips may have a thickness from about 0.002 inch (2.0 mil) to about 0.007 inch (7.0 mil). The foil strips may have an axial dimension of from about 8.0 cm to about 14.0 cm, and an overall length (prior to heat exchanger fabrication) of from about 90 m to about 110 m. The foils 56, 58 preferably are rolled together to fabricate a heat exchanger 50 in the shape of a cylinder having a diameter of from approximately 16 cm to approximately 30 centimeters. This mode of fabrication allows for a variety of heat exchanger diameters, however, and all dimensions are offered only by way of example. The axial width of the foil coil heat exchanger 10 corresponds to the axial width of the foil strips from which it is fabricated, e.g., preferably approximately 10 cm. The processor assembly 20 and exchanger 50 are fully scalable according to gas processing design demands.
The corrugated foil baffle 58 is fabricated so to present in the axial direction a series of alternating “peaks” and “valleys,” as seen in
Referring to
Referring to
The foil separator 56 in certain embodiments (such as might be used for gas heating to drive a Brayton Engine) where the processing temperature of the heat exchanger 50 does not exceed about 900° C. may be a thin copper foil, or nickel-coated copper foil. Copper has a comparatively lower melting temperate (approximately 1070° C.), but is a good conductor of heat energy to promote even heating of the exchanger 50. Likewise, under similar operating specifications, the corrugated foil baffle 58 may be pressed from a thin copper or nickel-covered copper foil. Nickel is generally more inert, but may also serve as a mild catalyst for certain hot gas processing reactions, so a nickel-coated foil 56 and/or 58 may be beneficial in some applications. However, where the heat exchanger 50 is to be solar heated to temperatures above 900° C., the foils 56 or 58 should be comprised of a superalloy. Thus, in a preferred embodiment of the apparatus adapted for use in gas processing, for example to reform methane or to drive a reverse water-gas shift reaction, the foil separator 56 and the corrugated foil baffle 58 are both made of superalloy, for example, Haynes® 214® nickel superalloy available from Haynes International, Inc., of Kokomo, Ind., USA. Such a superalloy has a melting temperature of about 1355-1400° C., permitting the exchanger 50 to be solar heated to adequate processing temperatures without physical failure.
Still referring to
As indicated by
Such varying of the characteristics of the baffle 58 as a function of radial distance from central axis may be accomplished by adjusting the compressive force applied to the foils 56, 58 during their spiral winding. At the inception of winding—that is, where the foils 56, 58 are first rolled to define the center of the exchanger 50—a relatively relaxed winding force and pressure is applied to the pair of adjacent foils 56, 58. Consequently, the baffle 58 maintains its original axial thickness (as corresponding generally to nearly the initial manufactured altitude p). The first three or four (for example) courses or windings of the foils 56, 58 are maintained with the originally applied winding compression and force, such that the distance between adjacent windings of the separator 56 remains unchanged at about the maximum (initial) distance. Then, and as suggested by evaluation of
Advantageously, using known physics formulae, the density of the exchanger coil can be selectively varied by selectively varying the winding force applied to the foils 56, 58 during rolled fabrication of the exchanger 50. Greater compression applied to the foils 56, 58 during rolled winding results in decreased baffle height p, as the baffles are “smashed” to reduce the absolute volume of the flow channels, which volume is a function of the baffle's axial profile. Normally, the exchanger 50 is roll fabricated to have its least dense foil windings at and near its central axis (to promote cooling gas flow axially past the foils 56,58), and with density increasing progressively as some predetermined function of the distance from the central axis. However, one skilled in the art will appreciate that the applied winding force can be varied in many modes during fabrication of the exchanger, so that coil density may be varied to either to decrease or increase at different distances from the axis. One skilled in the art also will design, by application of known principles of fluid dynamics, an exchanger 50 according to the present disclosure to adjust density to maximize the available heat transfer area presented by the exchanger, but without unduly compromising gas flow through-put.
Additionally, the density of the foil coil within the exchanger 50 affects the distance which incident solar rays are able to penetrate the interior of the exchanger 50. The less dense the foils 56, 58 are wound—that is, the greater the distance between windings of the foil separator 56 and the more “open” the corrugated baffle 58 -the further the solar rays are able axially to penetrate the exchanger 50. Increased axial penetration may promote a relatively more consistent radiant heating of the exchanger foil coils 56, 58 along the axial dimension, decreasing the steepness of the thermal gradient along the exchanger 50. Beneficial results are increased efficiency in feed gas heating and prolonged exchanger life. Notably, however, the foils 56, 58 are not to be wound too “loose,” as a critical factor in the function of the exchanger is the total area presented by the foils 56, 58 for contact with the passing gas. More and denser windings increase the available foil area for heat transfer to the gas, thus better driving the heat transfer equation, in which foil area is to be optimized, but in view of the need also not to unduly hinder gas flow.
Returning attention to
In the embodiment of
An advantage of the present apparatus is that the foil coil exchanger 50, while not only transferring convectively the solar energy into the flowing gas, can be operated in either a forward-flow or a reverse-flow flow mode. Thus alternatively, and as seen in
In the embodiment of
Reference is made to
The small directional arrows of
The apparatus and method works well for solar applications because its design transfers concentrated heat (bright solar spot) into a feed or process gas by providing a large gas contact surface area. The use of copper foils 56, 58 in the body of the exchanger 50 is effective, because its high conductivity spreads the heat from the very hot front surface 52 toward the back face 54 (that is not being directly irradiated). Consequently, the gas passing through the foil coil heat exchanger has copious surface for heat transfer. This is expressed by the convective heat transfer equation:
P=K A ΔT
where P is the thermal power transferred, K is the convective heat transfer constant, and ΔT is the temperature difference between the gas temperature and the temperature of the surface. The convective heat transfer constant K is a function of the type of gas and its velocity, and may be calculated using known concepts and formulae. The convective heat transfer rates typically are between 10 and 35 watts/m2 per degree K, depending on the gas, the surface, and the temperature.
From the heat transfer equation it can be determined that to transfer 50,000 watts at a ΔT of 100° K requires near 20 m2 of exchanger surface area. If a foil 56, 58 is 10 cm wide, near 100 meters of foil are required for this case (not 200 meters, because each foil 56, 58 has two sides).
We have operated a case where 44 kW of heat was transferred into flowing air. The foil coil exchanger was 10 cm wide, with the wound foils 56, 58 being 95 meters long. While it is difficult to assign a ΔT for such exemplary run, it is known that the temperature varied between 1060 degrees C. at it hottest point (center of the front face 52 to only about 400° C. at the rear face 54, near the outer edge). The exit gas was heated to an average temperature near 700° C. for the important part of the run. The homogeneity of the gas flow is unknown for the case, but its total flow rate was 147 SCFM.
It is of interest to look at the range of likely average ΔT and heat transfer parameters for the experimental run. If it is assumed that the gas flow was homogeneous, the product of KΔT would be near 2600 watts/m2. The following Table 1 shows compatible sets of K's and ΔT's for the example run. These are all in the probable range and verify the effectiveness of the disclosed foil coil exchanger design.
In order to explore the advantages of flowing the gas in against the thermal gradient in the exchanger (
The apparatus and method may be used, for example, to reform methane. The elevated temperatures obtained in the foil coil exchanger 50 are realized from solar energy, and are sufficiently high to drive the natural gas reformation step of the known steam methane reforming process. At high temperatures (700-1100 ° C.) and in the presence of a metal-based catalyst (e.g., nickel), steam reacts with methane to yield carbon monoxide and hydrogen. The appropriate feedstock gases are passed through the exchanger 50 according to the forgoing disclosure, and the reformation reactions thereby driven forward.
Some of the chemical reactions that can take place in the course of methane reformation are:
CnHm+n H2O→n CO+(m/2+n) H2
and
CO+H2O→CO2+H2
The produced carbon monoxide can combine with more steam to produce further hydrogen via the water gas shift reaction. Of course, other reactions (some undesirable, like coke formation) can take place if local conditions are favorable. The first reaction is endothermic while, the second reaction is exothermic. Additional fundamentals regarding methane reformation are found in, for example, U.S. Pat. No. 7,087,651 to Lee-Tuffnell, et al., and U.S. Pat. No. 6,312,658 to Hufton, et al.
The disclosed apparatus and method may also be adapted to exploit the Reverse Water-Gas Shift Reaction (rWGSR). The rWGSR reaction is given by equation:
CO2+H2→CO+ΔH2O H=+9 kcal/mole (38.9 kJ/mol)
The rWGS reaction accordingly may be exploited to generate CO from CO2. The CO may then be used as feedstock for further processing into useable fuels. For example, produced CO can be feed directly into a known Fischer-Tropsch synthesis system to generate synthetic fuels. The Fischer-Tropsch synthesis is a relatively complex network of both parallel and series chemical reactions; it is a carbon-chain building process whereby CH2 groups are attached to the carbon chain, and involves the catalytic reaction of H2 and CO to form hydrocarbon chains of various lengths. A principle by-product of the Fischer-Tropsch process system is water.
This reaction is endothermic, and occurs at relatively reduced temperatures in the presence of certain catalysts (e.g., element 59 in
The catalyzed rWGSR reaction always is most limited, however, by the forward water-gas shift reaction, as the reverse (or in this instance the “reverse-reverse”) reaction normally also is catalyzed. One possible mode for minimizing the back-reaction is to condense (and centrifugally separate) H2O from the flowing mix subsequent to partial reaction, followed by a recycling loop. (Separation of CO2 from the produced gas stream will always be necessary.) Further, the undesirable back-reaction can be suppressed by strategic placement of the catalysts within the reactor (forward location in the exchanger 50), coupled with rapid quenching by immediately downstream heat exchangers, such as for example, as disclosed in U.S. Pat. No. 7,140,181 to Jensen, et al., entitled “Reactor for Solar Processing of Slightly-absorbing or Transparent Gases.” While cooling the gas stream immediately after the rWGS Reaction shifts the equilibrium toward the promotion of CO2 and H2, at the reduced temperature the forward water-gas shift reaction is relatively slow, and in the absence of an immediately available and suitable catalyst may not occur at all.
Because in the endothermic rWGS Reaction the amount of equilibrium conversion of carbon dioxide into carbon monoxide is directly proportional to temperature, very high reactor temperature in the foil coil exchanger 50 is desirable. This makes the rWGSR an attractive candidate for the use of concentrated radiant energy to drive the thermochemical process.
The disclosed method and apparatus thus may be adapted to drive the rWGS Reaction. Concentrated sunlight, as from a suitable mirrored collection dish 10, is directed through the quartz window 28, into the processor assembly 20, and upon the foil coil heat exchanger 50. The spiraled and knurled foils 56, 58 in the heat exchanger 50 provides the heat transfer and also lends itself to providing desirable catalysis, such as by including Cu/alumina or Cu/silica, or CuO in combination with a Ni coating on foils 56, 58.
Hydrogen is mixed with carbon dioxide to produce an H2: CO2 feed gas mixture in a ratio of approximately 1:1. Alternatively, it may be desirable to “overdose” the H2, to boost the ratio to near 3:1 to reduce the amount of unconverted CO2 in a first pass. Other mixture ratios may be used as well to optimize the performance of the rWGS reactor.
As seen in
Quenching (rapid cooling) of the process gas is important for CO2 splitting. The quencher is merely a heat exchanger placed immediately downstream of the reaction (where radicals involved in the undesirable back-reactions, including especially the forward gas-water shift reaction, are favored) to cool the gas below about 450° C. and preferably to about 200° or so.
Accordingly, the produced CO and H2O may be vented through the forward port 30 and immediately quenched in a cooling exchanger (not shown) in the physical absence of any rWGSR catalyst materials. Heat surrendered from the product gasses during quenching can be used to provide pre-heating to the feedstock CO2 entering the input port, if desired.
The apparatus may be operated in a recycling mode, with close to 100% equilibrium conversion. Alternatively, “boosting” conditions can be provided, as previously explained, to drive the reaction to complete consumption of one of the reactants (e.g., CO2) by overloading the other (H2), followed by product extraction (preferably the produced water). The carbon dioxide may be separated by isothermal compression and further cooling or by other known methods. Produced CO2 preferably is returned to react in the processor 20 via the rear input port 32. The remaining carbon monoxide and hydrogen may be fed directly to a syngas processor. In an alternative process, the hydrogen may be separated (at least partially) from the gas mixture by membrane filtration, and returned as feedstock to the rWGS Reactor processor assembly 20.
Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all patents cited above are hereby incorporated by reference.
This application claims the benefit of the filing of U.S. Provisional Patent Application Ser. No. 61/127,034 entitled Low Entropy Heat Exchanger, Especially for Use with Solar Gas Processors, and Beam Spreader Useable Therewith, filed on May 10, 2008 and the entire specification thereof is incorporated herein by reference. Also, this application claims the benefit of the filing of U.S. Provisional Patent Application Ser. No. 61/197,922 entitled Solar Energy-driven Reverse Water-gas Shift Reaction for Generation of Syngas for Fuel Production, filed on Nov. 1, 2008 the contents of which also are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61127034 | May 2008 | US | |
| 61197922 | Nov 2008 | US |
